1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
2. Biodynamic Optical Imaging Center, School of Life Sciences, Peking University, Beijing 100871, P. R. China.
3. Program in Biophysics, Harvard University, Cambridge, MA 02138, USA.
4. Center for Reproductive Medicine, Third Hospital, Peking University, Beijing 100191, P. R. China.
5. Key Laboratory of Assisted Reproduction, Ministry of Education, Beijing 100191, P. R. China.
6. Peking-Tsinghua Center for Life Sciences, School of Life Sciences, Peking University, Beijing 100871, P. R. China.